The present invention relates to power supplies connected in parallel and, more particularly, to power supplies connected in parallel by the use of mosfets wherein the mosfets are used to monitor the current output of the power supplies and to isolate the power supplies from each other.
Many electronic devices, such as computers, have high power requirements at relatively low voltages. Accordingly, the power supplies associated with these devices are required to output relatively high current in order to meet the high power requirements. In order to achieve the high current requirements, the outputs of several power supplies are typically connected to each other in parallel. This parallel connection of the power supplies allows their current outputs to be summed and provides for redundancy. Accordingly, if one power supply fails, the system to which power is being supplied will be powered by the other supplies and will not fail.
The output of each power supply is typically connected to an electronic device or component that serves to isolate the power supplies from each other. For example, diodes may be connected between the outputs of the power supplies and the electronic device. When the power supplies are active, the diodes conduct current from the power supplies to the electronic device. In the event that one of the power supplies becomes inactive, the diodes associated with the inactive power supply ceases to conduct current. Accordingly, the inactive power supply is isolated and prevented from drawing current from the remaining active power supplies. The above-described isolation also prevents the voltage output of the remaining combination of power supplies from dropping due to the load of an inactive power supply.
In order to conduct the high current supplied by a power supply, the output of the power supply may have to be connected to several diodes that are connected in parallel. These diodes dissipate large quantities of heat when they conduct current, which requires provisions to convect the heat away from the diodes and other electronic components located in the vicinity of the diodes. For example, heat sinks may be physically attached to the diodes to convect heat from the diodes. In another example, the diodes may be mounted to a printed circuit board that has large amounts of copper in the area of the diodes to convect the heat away from the diodes.
In addition to the heat problem, the plurality of diodes occupies a large amount of area of a printed circuit board, which presents at least two problems. First, as electronic devices become more complex and smaller, the areas of their printed circuit boards are required to be small and to be occupied by as many components as is physically possible. The plurality of diodes and their associated convection devices occupy valuable area on the printed circuit board that could otherwise be occupied by electronic components. The second problem is that the large area of the printed circuit board occupied by the diodes will become excessively hot due to the number of heat dissipating diodes located in close proximity. As the printed circuit boards become denser with more electronic components, the electronic components in the proximity of the diodes and may be adversely affected by the excessive heat generated by the diodes. Accordingly, the excessive heat may adversely affect the operation of the electronic device as a whole.
Yet another problem with using diodes for isolation purposes is that their forward voltage drops are high relative to a low voltage power supply. For example, a schottky silicon diode typically has a forward voltage drop of about 0.4 volts. Therefore, a typical processing circuit requiring 3.3 volts is required to be powered by power supplies that output 3.7 volts in order to overcome the forward voltage drops of the diodes. In addition, the power supplies have to output more power in order to overcome the power dissipated by the diodes as a result of the forward voltage drops of the diodes.
A need exists for a power supply system that overcomes some or all of these problems.
The present invention is directed toward a power system comprising at least two power supplies operatively connected in parallel. The output of each power supply may be connected to an input of a switching device. The outputs of the switching devices may be connected together to provide the output of the power system. The switching devices may have controls, such as electrical inputs, that control the current flow between the inputs and the outputs. The switching devices may, as an example, be n-channel mosfets wherein the sources are connected to their respective power supplies, the drains are connected to the output of the power system, and the controls are the gates.
A voltage measuring device, such as a differential amplifier, may be connected across the input and the output of each switching device so as to measure the voltage drop across each switching device. The voltage measuring device may output a signal representative of this voltage drop. The signal may, as an example, be indicative of the polarity of the voltage drop between the input and the output of the switching device. The output of the voltage measuring device may be connected to the input of a comparator. The output of the comparator may be connected to the control of the switching device so as to control the current flow through the switching device.
When a power supply is active, the voltage at the input of its associated switching device is higher than the voltage at the output of its associated switching device. The voltage measuring device measures this difference and outputs a signal representative of this difference to the comparator. If the output of the voltage measuring device is greater than a preselected value, the comparator outputs a signal to the control of the switching device that causes the switching device to conduct current. The power supply is then operatively connected to the remaining power supplies and the output of the power system.
If a power supply becomes inactive, its output drops to zero volts. Accordingly, the polarity of the voltage drop across its associated switching device will change, which is measured by the voltage measuring device. The voltage measuring device outputs a signal to the comparator that is indicative of the reversed polarity. The signal will not be greater than the preselected value, so the comparator will output a signal to the switching device causing it to cease conducting current. The inactive power supply is, thus, isolated from the remaining active power supplies. In one example, the switching device is a mosfet, wherein the intrinsic body diode of the mosfet serves to isolate the inactive power supply from the remaining active power supplies.